Perhaps this is useful. The authors and institute can be written basically the same way, but use \\ instead of \par. The title needs to be written in a \parbox though, because of how \title is defined. If you want to avoid that \parbox, you'll need to add
\makeatletter
% redefine \title macro
\def\title#1{\gdef\@title{#1}}
\makeatother
to the preamble, before \title.
To place a logo with coordinates, you can make use of the fact that the whole poster is essentially a big tikzpicture, and that the position of the title can be obtained via the macros \titlepostop, \titleposbottom, \titleposleft and \titleposright. So you can use normal TikZ features to place the logo exactly where you like it relative to the title, e.g. with
\path (\titleposright,\titleposbottom) ++(-1cm,5mm) node[inner sep=0pt,above left] {\includegraphics{example-image}};
Place this right after \maketitle.
\documentclass[25pt, a0paper, portrait, margin=0mm, innermargin=15mm, blockverticalspace=15mm, colspace=15mm, subcolspace=8mm]{tikzposter}
\newcommand{\ini}{\textsuperscript{1}}
\newcommand{\inii}{\textsuperscript{2}}
\title{\parbox{\linewidth}{\centering The Journey of Walter Wombat and Other Wacky Creatures Meaning That the Title Will Span More Than One Line}}
\author{Carl Capybara\ini, Lazy Lizard\ini, Busy Bee\inii,\\
Gabrielle Giraffe\ini, Laura Lion\ini ~and Klaus Koala\inii}
\institute{\ini Institute of Applied Duck Dance\\
\inii Institute of Experimental Duck Dance}
\usetheme{Default}
\usetitlestyle{Empty}
\usecolorstyle[colorPalette=BrownBlueOrange]{Germany}
\begin{document}
\maketitle
\path (\titleposright,\titleposbottom) ++(-1cm,5mm) node[inner sep=0pt,above left] {\includegraphics{example-image}};
\block{Creating the document}{The document...}
\end{document}
Another option might be the example below. You can use Johannes' title code almost directly in \settitle, just with the addition of a \parbox.
\documentclass[25pt, a0paper, portrait, margin=0mm, innermargin=15mm, blockverticalspace=15mm, colspace=15mm, subcolspace=8mm]{tikzposter}
\newcommand{\ini}{\textsuperscript{1}}
\newcommand{\inii}{\textsuperscript{2}}
\usetheme{Default}
\usetitlestyle{Empty}
\usecolorstyle[colorPalette=BrownBlueOrange]{Germany}
\settitle{%
\parbox{\linewidth}{%
\centering
\color{titlefgcolor}
\vspace*{1em}
{\bfseries \Huge The Journey of Walter Wombat \par}
\vspace*{3em}
{\huge Carl Capybara\ini, Lazy Lizard\ini, Busy Bee\inii, \par Gabrielle Giraffe\ini, Laura Lion\ini ~and Klaus Koala\inii \par}
\vspace*{1em}
{\LARGE \ini Institute of Applied Duck Dance\par
\inii Institute of Experimental Duck Dance}
}
}
\begin{document}
\maketitle
\path (\titleposright,\titleposbottom) ++(-1cm,5mm) node[inner sep=0pt,above left] {\includegraphics{example-image}};
\block{Creating the document}{The document...}
\end{document}